Homogeneous ruthenium—phosphine catalysts are useful for the reduction of 1,2-dioxygenated organic compounds, such as alkyl oxalates, glycolic acid, and glycolate esters to ethylene glycol. In particular, catalysts containing ruthenium in combination with tridentate phosphorus ligands such as, for example, tris-1,1,1-(diphenylphosphino-methyl)ethane (also known as “triphos”), have been used for the reduction of glycolic acid to ethylene glycol. These catalyst systems, however, are expensive and their economical use requires efficient recovery of the metal and ligand from the reaction products.
Many metal-organophosphorus ligand catalysts are known to be sensitive to the composition of the gaseous phase above the catalyst reaction medium and to temperature. For example, the recovery of rhodium-phosphine hydroformylation catalysts often involve vacuum flashing, vaporization, or distillation of product from a nonvolatile catalyst composition that can result in decomposition of the phosphine ligand and precipitation of the metal component. Extractive methods of catalyst recovery, in which the catalyst components are extracted from the reaction effluent by an immiscible solvent, are known in the art but also suffer from similar disadvantages. The extracted catalyst composition typically must be concentrated by flash, vaporization or distillation away from the nonvolatile catalyst composition prior to reintroduction to the reaction zone with fresh feed substrate. These procedures often result in extended exposure of the sensitive catalyst components to damaging, elevated temperatures.
The thermal decomposition of catalyst components can be a difficult problem in the recovery of ruthenium-1,1,1-tris(diaryl- or dialkylphosphinomethyl)alkane catalysts used in hydrogenation of glycolic acid because of the high boiling points of the reaction products and complex array of reaction products and byproducts. A method for the efficient recovery of ruthenium-1,1,1-tris(diaryl- or dialkylphosphinomethyl)alkane catalyst compositions from glycolic acid hydrogenation product mixtures that reduces or avoids catalyst decomposition, therefore, would be highly desirable.